R1:972dfa7bfc37

Add 'hook.d/' directories to SVN and Git repositories for custom hooks

Summary:
Fixes T4189. Ref T4151. Allows repositories to have additional custom hooks for operations which can't be expressed with Herald (one such operation is lint).

This adds only local hook directories, since they're easier to use with existing hooks than global directories. I might add global directories eventually.

This doesn't support Mercurial since we have no demand for it and it's more complicated (we lose compatibility and…